1. Field of the Invention
This patent application pertains to an apparatus and method for testing blood constituents. More particularly, this application pertains to such apparatus and methods utilizing spectrophotometric analysis of blood constituents.
2. Description of the Prior Art
The use of spectrophotometric methods to quantitatively determine the concentration of a blood constituent are known. For example, U.S. Pat. No. 4,882,492 to Schlager teaches a non-invasive near-infrared measurement of blood analyte concentrations. The Schlager patent is particularly directed to the measurement of blood glucose levels. The Schlager patent recognizes that certain wavelengths of light in the near-infrared spectrum are absorbed by glucose. Modulated light is directed against a tissue (shown as an earlobe). The light is either passed through the tissue or impinged on a skin surface. The light is spectrally modified in response to the amount of analyte (for example, glucose) in the blood and tissue. The spectrally modified light is split with one beam passed through a correlation cell. The other beam is passed through a reference cell. The intensity of the beams passing through the correlation cell and the reference cell are compared to calculate a glucose concentration in the sample.
U.S. Pat. No. 4,805,623 to Jobsis teaches a spectral photometric method for quantitatively determining the concentration of a component in human blood. The Jobsis method teaches various steps including the determination of an apparent effective path length for the light which is being absorbed by the constituent being measured.
U.S. Pat. No. 4,655,225 to Dahne et al. teaches a spectrophotometric method and apparatus for non-invasive testing. The Dahne patent is particularly directed to the measurement of blood glucose.
U.S. Pat. Nos. 4,014,321 and 3,958,560 to March teach non-invasive glucose sensor systems which involve passing light through the cornea of the patient.
Notwithstanding the developments in the art, a need for an improved spectrophotometric measurement apparatus and method persists. For example, systems and methods which require the calculation of an apparent light pathway are susceptible to inaccuracy. Such a system is shown in the aforementioned U.S. Pat. No. 4,805,623. Systems which have fixed dimensioned light pathways (for example, U.S. Pat. 4,014,321) are restricted in their use and practicality. It is also desirable to develop a system and apparatus which can be used for non-invasive testing as well as invasive testing (for example, as a continuous monitor for testing blood glucose level during surgery or insulin treatment). Further, it is desirable to develop a system which can be used in conjunction with a chemical emission system (such as a blood glucose monitoring system which controls an insulin administering apparatus).